In the case of mounting fuel cells on a vehicle, a specific structure is required to protect the fuel cells from an external force. One proposed technique keeps the fuel cells in a fuel cell casing and mounts the fuel cell casing with the fuel cells kept therein on the vehicle (see, for example, Japanese Patent Laid-Open No. 2005-231549, No. 2003-123779, and No. 2003-182379).
In order to eliminate or at least reduce the influence of an external force applied to the fuel cells, there is a demand for enhancing the rigidity or the strength of the fuel cell casing for keeping the fuel cells therein. The proposed means for satisfying such a demand include a method of using a metal material for the fuel cell casing and a method of constructing the fuel cell casing to have a thick wall. The former method, however, undesirably increases the weight of the fuel cell casing, while the latter method undesirably increases the dimensions of the fuel cell casing. The increase of the weight and the increase of the dimensions of the fuel cell casing lead to an increase in total weight of the vehicle and a decrease in mounting space of the fuel cells, respectively.